1. Field of the Invention
The present invention is generally related to plating systems, and more in particular, continuous plating systems.
2. Description of the Related Art
Conventional approaches for plating metals onto portions of targeted parts can involve either a generally more precise step-and-repeat plating for plating selective locations or a generally faster continuous strip plating for plating portions of parts whose intended plating areas can be arranged in an uninterrupted path. Such targeted parts can be found joined together in a lead frame. A lead frame can be formed as a long continuous strip containing duplicate copies of a particular part. The lead frame can be fed through machines to perform various physical manipulations on each of the parts of the lead frame in an orderly stepwise fashion so that each of the parts has its turn to undergo each of the physical manipulations performed by the machines.
With the conventional step-and-repeat plating process, a precise mask of finite length is positioned over a section of a lead frame having a series of parts. The mask has various openings so that for each part of the masked lead frame section, those portions of the part that are to be plated remains unmasked. The unmasked portions of the parts of the masked lead frame section are subsequently exposed to a plating solution to accomplish their plating.
The lead frame is typically negatively charged to plate those exposed areas of the lead frame when they receive plating solution such as by pouring, spraying, or brushing the solution from a positively charged applicator, such as a nozzle. After the unmasked portions of the parts of the masked lead frame section have been plated, a new section of the lead frame is moved to be masked and to further repeat the step-and-repeat process.
Some implementations of the step-and-repeat process involve perforated tape masks that are each used for a portion of the lead frame and are subsequently removed and discarded after plating of the respective portion of the lead frame is accomplished. Although the step-and-repeat process can be used for precision plating so that relatively little plating material is wasted, the process can be inherently slow and labor intensive.
With a conventional continuous plating system, the lead frames can be run at constant velocity through the plating system therefore potentially reducing labor requirements and potentially increasing throughput. Conventional continuous plating systems include a de-reeler, welding apparatus, tanks, guide devices, masking belt and a re-reeler. The de-reeler feeds the lead frame from a first reel into the plating system. The welding apparatus includes a spot welder and a welding fixture to attach subsequent reels of lead frames to be fed into the plating system.
The tanks includes a series of cleaning, plating and washing tanks to electroplate nickel, gold or other precious metals on to portions of the parts of the lead frame. The guide devices are fashioned to direct the lead frame through the tanks while trapping the parts between the moving masking belts. The masking belt exposes one or more portions of each part of the lead frame through one or more openings in the masking belt to plating solution to be plated and covers other portions of each part to prevent those portions from being plated.
The re-reeler spools the plated parts onto a second reel as the parts emerge from the plating system. Within reason, the longer the portion of the lead frame that can be exposed to the plating solution at any one time, the faster the lead frame can be run through the continuous plating system and consequently, the faster the throughput of the continuous plating system.
Although conventional continuous plating systems can have relatively faster throughput than the conventional step-and-repeat plating systems, there is a price for this faster throughput with conventional continuous plating systems. Namely, conventional continuous plating systems tend to be more wasteful of the plating materials.
A problem exists with conventional continuous plating systems in that the masking belt typically shifts back and forth in position orthogonal to its direction of motion, also referred to as trans-linear motion. This trans-linear motion causes a shift back and forth in position of each of the openings in the masking belt relative to its associated lead frame part to be masked. The trans-linear back and forth shifting of position of the opening thus creates uncertainty as to where the opening will be positioned with respect to the particular lead frame part at the point when plating of the part occurs.
Consequently, if the opening was only as large as its corresponding desired portion of the part to receive plating, this desired portion of the part may not be fully plated. Through the trans-linear shifting, the opening may not be properly positioned over the part at the time of plating. Rather, the opening may be somewhat out of position and if the opening was only the size of the desired portion of the part to be plated, not all of the desired portion of the part would be exposed through the opening to receive the plating solution.
Thus, to compensate for this shifting due to the trans-linear motion, each of the openings are enlarged enough so that no matter where an opening is in its back and forth trans-linear motion, all of the desired portion of the part to be plated is still exposed through the opening to receive the plating solution. This compensation, however, has a price. Since the opening in the masking belt is larger than the desired portion of the part to be plated, areas of the part that do not require plating will be resident in the area of the opening and will be plated, which wastes the plating metal.
This enlargement of the opening larger than the desired portion of the part to be plated is referred to as over-plating. With some conventional implementations of continuous plating systems an over-plating of 0.06 inches on either side of the desired portion of the part to be plated is not an unusual value.
Some conventional implementations of the continuous plating system may try to use masking belts of relatively greater thickness to possibly reduce the amount of trans-linear motion. A drawback of increased masking belt thickness is a result referred to as a wall effect in which the mask thickness inhibits the thickness of the plating near the edge of the opening. The combination of an opening of the belt mask with one of the lead frame parts forms a sort of canyon with the part acting as sort of the floor of the canyon and the edges of the opening acting somewhat like vertical walls of the canyon.
As the lead frame part is pressed with the mask belt and is moved through the tank of plating solution, the plating solution enters the so called canyon, but interaction of the plating solution with the walls of the canyon somewhat hinders the plating solution from depositing plating material to as great an extent on the floor of the canyon (a portion of a part of the lead frame) near the walls of the canyon (edges of the opening) when compared to the amount of plating material deposited farther away from the walls.
As a result, the plating material on a plated portion of a part has an uneven thickness, being thinner near the edges of the plated portion and being thicker near the center of the plated portion. Generally speaking, the thicker the belt mask, the larger the difference in thickness is between the edges and the center of a given plated portion. Resultant uneven plating can also waste plating material because more plating material may need to be used in a center of a plated portion in order to have a sufficient amount of plating material near the edges of the plated portion.
The cost of plating precious metals is primarily determined by the amount of gold or other precious metal used and the throughput speed involved. The two conventional approaches discussed raise costs by either being relatively slow and labor intensive, or by being relatively wasteful of plating material.